1. Field of the Invention
The invention relates to a method and an apparatus for the measuring of structures in a fingerprint or the like, comprising the measuring of chosen characteristics of the surface of the fingerprint, e.g., capacitance or resistivity, using a sensor array comprising a plurality of sensors, positioned in contact with, or close to, the surface.
2. Description of the Related Art
Identification by the use of fingerprints has lately come to the fore as a result of the increasing needs for security relating to, for example, credit cards or computer systems, as well as the greatly increased availability of pattern recognition algorithms. Some systems for recognition of fingerprints have already been made available on the market. The techniques used to register the fingerprint varies.
Some of the previously known solutions are based upon optical technology using light with one or more wavelengths. These are sensitive to dirt and contamination, both in the fingerprint and on the sensor surface, and thus cleaning is necessary for both.
Another alternative is pressure measurement, such as is described in U.S. Pat. No. 5,559,504, U.S. Pat. No. 5,503,029 and U.S. Pat. No. 4,394,773. This, however, has the disadvantage that the sensor surface becomes sensitive to mechanical wear and damage, as the sensor has to have an at least partially compliant surface.
Temperature sensors have also been suggested, for example in U.S. Pat. No. 4,429,413 and international patent application PCT/NO96/00082.
Since fingerprint sensors may be exposed to long term use in varying and sometimes demanding conditions the sensor needs to have a robust surface and to be as insensitive to pollution in the fingerprint and on the sensor as possible. It must be capable of reading most fingerprints without being disturbed by latent prints from earlier use. In some cases, e.g., in credit cards or computer keyboards, it would also be advantageous if the sensor could be made compact.
In the view of costs there is also a demand for simplicity and minimizing of the number of parts.
In addition to the solutions mentioned above, the measuring of capacitance has been tried as a method to measure fingerprints. Examples are shown in U.S. Pat. No. 4,353,056 and U.S. Pat. No. 5,325,442. While the ridges of the fingerprint touches the sensor surface, the valleys have a small distance to the sensor surface, resulting in a difference in capacitance and/or conduction measured at the different sensors. Humidity may affect the measurements, but if it is even throughout the fingerprint an analysis of the contrast between the measurements can provide a picture of the fingerprint.
All the solutions mentioned above are based upon two-dimensional sensor arrays with dimensions comparable to the size of the fingerprint. These are expensive and difficult to produce, since they comprise a large number of sensors simultaneously measuring the surface.
EP 735,502 describes the use of a one or two-dimensional array of sensors being moved in relation to the fingerprint. The described solution is based on the measuring of resistance, and has a limited resolution defined by the minimum sensor dimensions and the distance between the sensors.